The invention relates to substantially reducing fluid loss in a low toxicity oil-base drilling mud by adding an effective amount of a conjugated diene-vinylaromatic-conjugated diene block polymer containing about 20% by weight or more styrene to the drilling mud to produce a drilling fluid composition wherein the fluid loss is equal to or less than 0.2 ml/30 minutes according to the standard testing methods referenced herein.
Drilling fluids are used in the process of drilling bore holes in subterranean deposits such as gas and oil. The boring is accomplished by well drilling tools and a drilling fluid. Drilling fluids serve to cool and lubricate the drill bits; to carry the cuttings to the surface as the drilling fluid is circulated in and out of the well; to support at least part of the weight of the drilling pipe and drill bit; to provide a hydrostatic pressure head to prevent caving in of the walls of the well bore, to deposit on the surface of the well bore a filter cake which acts as a thin, semi-pervious layer to prevent undue passage therethrough of drilling fluids; and to perform other functions as are well-known in the drilling art. It may be desired that the drilling fluid exhibit a relatively low rate of filtration or fluid loss in addition to having desirable, rheological properties, such as viscosity and gel strength.
The drilling fluid includes an oil component. It is conventional to use diesel oil, but low toxicity synthetic and non-synthetic oils are presently being used in place of diesel oil.
In one aspect, this invention relates to a method for improving the fluid loss in a low toxicity oil based drilling mud oil (LTOBM).
In general, in one aspect, the invention provides a drilling fluid composition comprising an LTOBM oil. About 0.05% to about 2.0% by weight of the drilling mud oil is a fluid loss agent, which comprises a butadiene-styrene-butadiene block polymer (BSB) block copolymer having about 20% by weight or more styrene. The fluid loss characteristic of the drilling fluid composition is generally equal to or less than 0.2 ml/30 minutes.
The other characteristics of the present invention will become apparent from the further disclosure of the invention, which is given hereinafter.
One embodiment of the invention is directed to adding 0.05 to 2.0% by weight of a BSB block copolymer containing about 20% by weight or more styrene to an LTOBM to produce a drilling fluid exhibiting reduced fluid loss equal to or less than 0.2 ml/30 minutes. In another embodiment of the invention, the BSB block copolymer contains about 25% to about 50% by weight styrene.
The BSB block copolymer is a tri-block polymer. The most common tri-block polymers have a poly(vinylaromatic) segment such as styrene at the ends of the molecule, and an elastomeric segment such as a conjugated diene in the center of the block polymer. Examples of such tri-block polymers include styrene-butadiene-styrene (SBS). The BSB tri-block polymer is not a common block copolymer as is SBS because it has poor physical properties in the solid form. However, it has been discovered that such block copolymers enhance the fluid loss property of a drilling fluid when added to an LTOBM.
The xe2x80x9cBxe2x80x9d segment of the BSB block polymer is a diene polysegment which can be a polymer from conjugated diene monomers having 4-6 carbons atoms per molecule such as 1,3-butadiene, isoprene, 2-ethyl-1,3-butaiene, 2,3-dimethyl-1,3-butadiene and piperylene. The xe2x80x9cSxe2x80x9d segment of the block copolymer is a monovinyl aromatic polysegment. Examples of such are styrene, xcex1-methylstyrene, p-vinyltoluene, m-vinyltoluene, o-vinyltoluene, 4-ethylstyrene, 3-ethylstyrene, 2-ethylstyrene, 4-tert-butylstyrene and 2,4-dimethylstyrene.
In the context of the present invention, an LTOBM is a mud composition that does not contain carcinogenic components. In some cases, LTOBMs may be selected according to additional considerations for enhanced environmental friendliness. LTOBMs are generally considered safer than those incorporating conventional diesel oils. Examples of LTOBMs include those containing synthetic drilling mud oils such as ESCAID(copyright) 110 (Exxon Mobile Corp.), NOVAPLUS(copyright) (from M-1 Drilling Fluids L.L.C.), SARALINE(copyright) (Unical Corp), or a non-synthetic oil such as mineral oil.
Drilling fluids may contain additives and conditioning agents, which are important in determining the fluid loss properties of the drilling fluid, as well as inhibiting shale and clay disintegration. As an example, such additives or agents may include modified lignite, polymers, oxidized asphalt, gilsonite, and humates prepared by reacting humic acid with amide or polyalkyl polyamines.
A drilling fluid of the present invention is used in combination with a rotating drill bit to drill a borehole in a subterranean formation. An exemplary drilling method comprises the steps of rotating a drill bit in the borehole and introducing the drilling fluid into the borehole to pick up the drill cuttings and carrying at least a portion of the drill cuttings out of the borehole. The drilling system employed in such method comprises the subterranean formation, the borehole penetrating the subterranean formation, the drill bit suspended in the borehole, and the drill fluid located in the borehole and proximate the drill bit.
The drilling fluid composition of the invention comprises LTOBM as described above and a BSB block polymer as the fluid loss agent. In some embodiments, the composition may include, in any combination, (i) weighting agents such as barite, hematic, calcium carbonate, galena, siderite and mixtures thereof, to adjust the density of the drilling fluid; (ii) an organophilic clay such as hectorite, bentonite and mixtures thereof as a viscosifier and gelling agent; (iii) lime; and (iv) emulsifiers and wetting agents such as surfactants, ionic surfactants such as fatty acids, amines, amides and organic sulphonates and mixtures thereof.
In various examples discussed below, the fluid loss agent is a block copolymer having a BSB architecture. Such polymers can be made using the same anionic polymerization techniques employed to prepare SBS block polymers. Styrene and 1,3-butadiene are dried by passage over activated alumina (Kaiser A-201), and then copolymerized and coupled in a five-stage process using n-butyllithium initiator. Polymerization is conducted under nitrogen in a stirred, jacketed, stainless steel reactor having a capacity of 7.6 liters employing essentially anhydrous reactants and conditions. The anhydrous mixtures are stirred continuously during the polymerization process. The process is similar to that used to prepare SBS block copolymers as described in U.S. Pat. No. 4,584,346 which is incorporated herein by reference.
The polymerization is carried out in solution in an inert organic hydrocarbon diluent. Suitable hydrocarbon diluents include aliphatic, cycloaliphatic or aromatic hydrocarbons, which are liquid under the reaction conditions and are preferably of 4 to 12 carbon atoms. Examples are isobutane, n-pentane, isooctane, cyclopentane, cyclohexane, cycloheptane, benzene, toluene, the xylenes and others. Mixtures of these solvents may also be employed. Furthermore, the polymerization is carried out in the presence of small amounts of ethers such as tetrahydrofuran (THF), dimethoxyethane, phenyl methyl ether and others, whereby it is possible to influence, in the conventional manner, the rate of polymerization, the configuration of the conjugated diene polymer segment, and the statistical transition between the conjugated diene and styrene segments.
The amount of the initiator employed in the first stage of the process may depend on the desired molecular weight of the polymer. The initiators employed are the conventional monolithium-hydrocarbons of the general formula RLi, where R is an aliphatic, cycloaliphatic, aromatic or mixed aliphatic-4 aromatic hydrocarbon radical, which may be of 1 to about 12 carbon atoms. Examples of the lithium hydrocarbon initiators to be employed according to the invention are methyllithium; ethyllithium; n-, sec- and tert-butyllithium; isopropyllithium; cyclohexyllithium, t-phenyllithium and p-tolyllithium. The monolithium-alkyl compounds where alkyl is of 2 to 6 carbon atoms are preferred, n-butyllithium being particularly preferred.
The polymerization process is a five-stage process. In the first stage, the polymerization is the virtually complete conversion of a conjugated diene monomer to a polymer to form a diene polysegment. The polymerization is carried out in a hydrocarbon diluent in the presence of an initiator. In particular, the reactor is charged with a mixture of cyclohexane and about 0.04 to 0.2 parts by weight per 100 parts of total monomer (phm) of THF preheated to about 50xc2x0 C., then 30 to 40 phm 1,3-butadiene and 0.01 to 0.025 phm of n-butyllithium initiator (2% by weight solution in cyclohexane) are added, after which an additional amount of cyclohexane is added to flush the lines.
In the second stage of the process, a monovinyl-aromatic compound is polymerized onto the active chain end of the diene polysegment to form polysegment having an active vinyl-aromatic chain end. In particular, after the polymerization to make the butadiene segment, the reactor is charged with 30 to 40 phm styrene to form a styrene polysegment attached to an end of the butadiene segment leaving an active styrene chain end, after which the lines are rinsed with cyclohexane.
In the third stage of the process, a conjugated diene monomer is polymerized onto the active chain end of the monovinyl-aromatic polysegment. In particular, 30 to 40 phm of butadiene is added to the reactor, followed by cyclohexane to flush the lines.
The fourth stage of the process is the chain termination step where water and carbon dioxide are introduced into the reactor to terminate the polymerization.
The fifth and final stage of the process is a stabilization step wherein antioxidants are added to the reactor. The antioxidants used comprise a hindered phenol such as tetrakismethylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate) methane (a commercial brand of this stabilizer is IRGANOX 1010 from Ciba) in addition to an organic phosphite such as trisnonylphenyl phosphite (TNPP, obtained from GE Specialty Chemicals). The antioxidants are incorporated into the polymer by charging equal amounts of a 2% solution of IRGANOX in THF and a 5% solution of TNPP in cyclohexane to the reactor after the addition of the carbon dioxide.
Following the stabilization step, each copolymer solution was flashed at 178xc2x0-180xc2x0 C. to remove a portion of the cyclohexane diluent. Substantially all of the remaining diluent was removed in a vacuum oven by drying at 90xc2x0 C. for one hour. The resulting polymer was easily crumbled by hand and then dried for an additional hour in a vacuum oven.
The concentration of the block copolymer in the low toxicity drilling mud is in the range of about 0.05% to about 2.0% by weight of the drilling fluid, preferably about 0.075% to about 1.5% by weight of the drilling mud, and more preferably about 0.1% to about 1.0% by weight of the drilling fluid. As an example, it may be desirable to have the styrene content of the block copolymer greater than 25%, and the fluid loss reduced to a level that is equal to or less than 0.2 ml/30 minutes.